1. Field of the Invention
The present invention relates to a thermal infrared detector in which infrared rays are detected by conversion to heat and to an infrared focal plane array including such sensors aligned in a two-dimensional arrangement, and particularly to a structure of a thermal infrared detector detecting infrared rays with high sensitivity and low noise.
2. Description of the Background Art
Infrared focal plane arrays catch sight of objects that do not stimulate human sight and instantly measure temperatures of objects from distant places without contact. Such arrays are being used in diverse ways in a wide variety of industrial fields, for measuring and controlling manufacturing lines, as medical and diagnostic devices, and devices detecting humans. A general infrared focal plane array includes infrared detectors arranged in a two-dimensional matrix, and a signal read-out circuit around the array for reading signals from the detectors.
Infrared detectors that are used in infrared focal plane arrays may be generally classified by their principles: those of the quantum type in which infrared rays as photons are utilized; and those of the thermal type in which thermal actions of infrared rays are utilized. While the quantum type have an advantage in high sensitivity and rapid response, the entire device is complicated and costly since detectors need to be cooled to around −200° C. The thermal type exhibit inferior response speed, but they need not be cooled and may operate at room temperature. Therefore, infrared focal plane arrays employing thermal type infrared detectors are the trend in general purpose use.
In thermal type infrared detectors, changes in temperature caused through absorption of infrared rays irradiated from a material are converted into electric signals by means of temperature sensors in which electric characteristics are changed through temperature. Among known thermal type infrared detectors that have been developed in the past, there are some that employ resistors (that is, resistance bolometer films), which ohmic values changed in accordance with temperature, as temperature sensors (for instance, U.S. Pat. No. 5,286,976, Japanese Patent Laid-Open Publication No. 5-206526, U.S. Pat. No. 6,144,030), or some which employ semiconductor elements such as diodes or transistors as temperature sensors (for instance, U.S. Pat. No. 5,977,603, U.S. Pat. No. 6,465,784, Japanese Patent Laid-Open Publication No. 2000-321125, T. Ishikawa, M. Ueno, K. Endo, Y. Nakaki “Low Cost 320 by 240 Non-cooling IRFPA Employing Conventional Silicon IC Processes”, Part of the SPIE Conference on Infrared Technology and Applications XXV, USA, April 1999, SPIE Vol. 3698, pp. 556–564). Since such thermal infrared detectors may be formed on semiconductor substrates made of, for instance, mono-crystalline silicon, detectors and signal read-out circuits may be advantageously formed in a simultaneous manner within the same semiconductor manufacturing lines.
In general thermal type infrared detectors in which bolometer films are employed as temperature sensors, bolometer films that are to serve as the temperature sensors are of bridge arrangement in which they are lifted upward of the substrate by means of thin film supporting legs exhibiting high thermal resistance (for instance, FIG. 1 of U.S. Pat. No. 5,286,976 or FIG. 2 of Japanese Patent Laid-Open Publication No. 5-206526). Bolometer films are heat-insulated with respect to the substrate through the supporting legs while they are electrically connected with a signal read-out circuit formed on a substrate downward of the bridge through wirings within the supporting legs. Due to this arrangement, when infrared ray enters thereto, the change in temperature of the bolometer films is caused and the resistance of the bolometer films is changed owing to changes in temperature. The change in the resistance may be output as changes in voltage or current.
Among thermal type infrared detectors employing semiconductor elements such as diodes or transistors as temperature sensors, some are known that diodes or like are formed on a polycrystalline silicon film grown on semiconductor substrates (for instance, U.S. Pat. No. 5,977,603) or that diodes are formed on an. SOI substrates (for instance, Ishikawa et al., Part of the SPIE Conference on Infrared Technology and Applications XXV, USA, April 1999, SPIE Vol. 3698, pp. 556–564). In both cases, signal read-out circuits and temperature sensors may be formed simultaneously in ordinary semiconductor processes so that it is possible to obtain infrared detectors suitable for mass production at high yields.
When forming temperature sensors such as diodes on to polycrystalline silicon films, the diodes that are to serve as the temperature sensors will be arranged in a bridge-like conformation similarly to a bolometer type in which the diodes are lifted upward of the substrate through supporting legs having high thermal resistivity (see FIG. 1 of U.S. Pat. No. 5,977,603). When forming temperature sensors such as diodes onto mono-crystalline thin films of SOI substrates, the temperature sensors will be arranged such that they are supported by supporting legs, which are formed by hollowing out a part of the substrate under the insulating thin film.
In an infrared focal plane array in which such thermal type infrared detectors are arranged two-dimensionally as pixels, the size of each single infrared detector is limited. The fill factor, which is the area ratio of an infrared absorbing region occupying a pixel area (that is, area of respective infrared detectors), should be made as large possible for the purpose of achieving high sensitivity. On the other hand, for improving the sensitivity with respect to infrared rays, it is also necessary to make the supporting legs long enough to improve heat-insulating characteristics of the temperature sensors. For this purpose, various methods have been studied for achieving high sensitivity of thermal type infrared detectors by improving the fill factor and securing a height for the supporting legs.
For instance, U.S. Pat. No. 6,144,030 discloses a two-layered arrangement in which a bolometer film and an infrared absorbing film are integrally formed as a broad region and thin film supporting legs of high thermal resistance extend downward. This structure is effective for bolometers since it is possible to secure the fill factor while simultaneously making the supporting legs longer than usual.
According to U.S. Pat. No. 6,465,784 and Japanese Patent Laid-Open Publication No. 2000-321125, fill factors of the infrared detectors are improved by employing an arrangement in which an infrared absorbing film, which is thermally connected to diodes that are to function as temperature sensors, is hanging over the supporting legs in a parasol-like manner.
In U.S. Pat. No. 5,760,398, fill factors are substantially improved by providing a wide concave mirror under the temperature sensors and the supporting legs, wherein infrared rays reflected by the concave mirror are focused onto the temperature sensors (see FIG. 2 and FIG. 3 of U.S. Pat. No. 5,760,398).